A variety of methods and apparatus have been marketed and/or proposed for securing together overlapping portions of thermoplastic strap, especially thermoplastic strap that has been encircled about an article or articles to form a loop and that has then been tensioned.
Conventional thermoplastic strapping material includes nylon, polypropylene, and polyester. Overlapping portions of strap formed from such materials may be joined together with suitable sealless techniques, including techniques employing bonding agents (e.g., adhesives) or welding (e.g., conventional hot knife or friction-fusion welding).
Regardless of the manner in which sealless joints are created between overlapping portions of thermoplastic strap, the strength of the joint is, obviously, of great importance. Such joints may not be as strong as the strap itself. The joint must have sufficient tensile strength to resist forces acting on the weld in the direction of the strap length. The joint must also have sufficient peel strength to resist forces which tend to act through the strap on the joint generally obliquely or perpendicularly to the length of the strap.
In some situations, a relatively low peel strength is desired. For example, a strap loop around an article may be easily removed if the strap joint can be readily peeled open. On the other hand, an excessively low peel strength can present problems where unintentional opening of the joint is not desired. For example, if an end of the outermost overlapping portion of strap is snagged during the handling of the strap or of article bound thereby, then the joint may peel open.
With welded joints, the peel strength may be considerably less than the tensile strength of the joint. Further, the problem of peel strength joint failure is more prevalent for thicker strap than for thinner strap.
It would be desirable to provide apparatus for providing an improved joint between overlapping thermoplastic strap portions--especially a joint with greater resistance to peel forces.
Further, it would be advantageous to provide apparatus for providing an improved configuration of lengths of flexible thermoplastic strap so as to accommodate the formation of an improved joint which has been formed previously or which may be formed simultaneously with, or after, the use of the apparatus.
It would be especially beneficial if such apparatus could be readily used with, or incorporated in, existing or new strapping tools and machines employing friction-fusion welding principles of operation because a variety of friction-fusion strap welding tools and machines have been successfully introduced over the years and are in widespread commercial use around the world.
Improved joints between overlapping lengths of flexible thermoplastic strap, and methods for producing the improved joints, are disclosed in the copending U.S. patent application Ser. No. 07/347,569 entitled "Improved Strapping Joint And Method For Forming Same" filed on May 5, 1989 by Nelson Cheung, Robert J. Nix, and Janusz P Figiel and assigned to the assignee of the present application. FIG. 5 in that copending patent application illustrates a welded joint wherein, prior to tensile loading of the joint area, the overlapping strap portions are deformed in a Z-like configuration by laterally relatively displacing the non-welded portions of the strap to lie on opposite sides of the then angularly disposed, planar connecting weld. The trailing portions of the strap are preferably parallel to each other. The description of the joint illustrated in FIG. 5 of the above-identified U.S. patent application and the description of the method of forming it are incorporated herein by reference thereto to the extent that such disclosures are not inconsistent with the present disclosures.
It has been found that by forming the strap so that the non-welded portions of the strap lie on opposite sides of the angularly disposed planar weld, there is an elimination or reduction of the force component which would induce peeling failure (i.e., separation) when the non-welded strap portions are loaded in tension.
This improvement results if the welded joint between the two overlapping strap portions is formed as discussed above substantially simultaneously with the formation of the welded joint. However, it is contemplated that the improved configuration could be created after a conventional joint is formed. Also, it is contemplated that the unique configuration could be formed in the overlapping strap portions before the weld per se is made.
It would be desirable to provide apparatus for producing such a configuration, or functionally equivalent configuration, in overlapping strap portions in a joint region.
Further, it would be beneficial if other embodiments of such apparatus could provide other improved configurations of the strap portions for accommodating the formation of a joint with even greater resistance to peel forces and for readily accommodating the formation of such a joint in conjunction with conventional friction-fusion welding techniques.